Reference is made to my related Disclosure Document No. 82,906 filed July 25, 1979 in the U.S. Patent and Trademark Office.
There is a widespread belief in the satellite communications industry that it is impossible to receive a useable signal from the typical communications satellite (broadcasting at 4 GHz with orbital spacing of 4 degrees) using small diameter (on the order of 2 feet) dish antennas and receivers having average noise figure (on the order of 2 dB).
Two primary problems and a host of minor problems give support to this view. The lack of directivity of small diameter antennas is one significant problem. Typically, geostationary satellites are spaced in orbits 4 degrees apart. The 9 degree half-power beamwidth of the typical 2 foot dish antenna at 4 GHz results in the reception of signals from adjacent satellites as well as the satellite of interest. The 9 degree beamwidth also results in a lower antenna gain at the desired frequency than that available with large diameter dishes. For example, a 2 foot dish at 4 GHz has an antenna gain of around 26 dB, while a 16 foot dish has an antenna gain of around 44 dB. At 4 GHz an antenna gain of 26 dB is too low to permit satisfactory detection of standard radio transmissions.
The second problem is the limit imposed by official regulatory bodies such as the United States' Federal Communications Commission (FCC), on satellite signal flux densities incident on the earth's surface in order to protect terrestrial communications links. When standard satellite modulation techniques (e.g. QPSK, FSK, BPSK, or PCM) are used at the permitted levels the signal level received when using a 2 foot dish antenna is far below the receiver thermal noise floor.
Other problems include the high cost of a system capable of handling a variety of data rates, intermittently used by any one of a large number of users, all requiring quick access time. Additionally, use of small modulation bandwidths, i.e. low data rates, may result in obliteration of data by such effects as carrier drift and phase and amplitude noise.
In the past, efforts to overcome these problems have focused on antenna or receiver design. The larger the antenna diameter, the larger the signal collecting area, therefore, the lower the signal levels which may be satisfactorily received. However, with increased antenna size, there is a corresponding cost increase. Likewise, development of precision antennas (i.e., antennas having precise shape, diameter and surface characteristics at the intended operating frequency) enhances antenna performance but also raises system costs. Focus in receiver design has been directed toward obtaining low noise contribution, also resulting in higher system costs.
The present invention solves the above problems by use of novel apparatus employing spread spectrum techniques as a means to enhance the effective gain and selectivity of the system without resort to the above special antenna designs or extra low noise figure receivers.